The goals of this proposal are to understand the mechanisms of viral entry into cells in the polyoma virus-mouse system. Important determinants of virus pathogenicity have been mapped to VP1, the major viral capsid protein. Attachment of virus to target cells in the host are mediated by unknown glycoproteins with specific sialic acid linkages. We will attempt to identify the cell attachment proteins that mediate virus entry, using several approaches. Mouse cell lines lacking functional receptor(s) will be identified on the basis of being resistant to viral infection but susceptible to viral DNA transfection. These lines will be transfected with cDNA expression libraries prepared from mouse tissues that are rich in receptors(s), and selected for virus susceptibility. To permit a direct positive selection for receptor- positive transfectants, a polyoma DNA vector encoding a Hygromycin- resistance gene in the place of VP1 will be constructed. Packaging of this vector will be attempted using a Sindbis-VP1 helper virus. Immunological approaches will be used involving screening for rat-anti- mouse cell monoclonal antibodies that protect against viral infection. Site-directed mutagenesis on regions of VP1 likely to be involved in cell binding and entry will be carried out. Finally, histological and biochemical approaches will be pursued to identify a class of natural inhibitors, or pseudoreceptors, in the mouse that prevent spread by certain strains of polyoma.